Project a photo onto a surface
I would approach it like this:

Find out what camera sensor size and lens where used to create the picture.

Create a camera in blender that matches those characteristics.

Add an image as background, and set it to Front with an opacity of 50% (so that the image is always present over the model)

Then activate Lock Camera to view, move the camera until you find the spot where the photo was taken from using the background image as a guide. 

Once you find the location and rotation where the photo was taken from, use the UV project modifier on the landscape object and set the camera as projector for the texture.

Note that this is not an easy task in any way.